This invention relates to AM stereophonic receivers, and particularly to AM stereophonic receivers for receiving signals modulated according to the well known independent sideband (ISB) modulation technique. However, the invention is also useful in receivers for other composite amplitude and angular modulated stereo signals wherein a demodulated AM signal is used in conjunction with the angular modulation of such a composite signal for stereo reproduction.
U.S. Pat. No. 4,018,994 to Kahn describes an AM stereophonic receiver which includes an envelope detector, for detecting the amplitude modulation component of a received ISB AM stereo IF signal, and circuitry for detecting the quadrature modulation resulting from the phase modulation in the IF signal. The particular quadrature demodulation circuitry used by Kahn modifies the quadrature modulation component of the IF signal prior to quadrature detection for purposes of distortion correction.
In AM receivers, and particularly those with continuous tuning, it is often useful to have a non-flat automatic gain control (AGC) characteristic so that the signal level at the output of the envelope detector continuously increases with increasing received signal strength, rather than level off at some point. This characteristic is illustrated in FIG. 1, which is a graph of the output signal level from an envelope detector as a function of received signal level. If an ideal AGC circuit is used, the output signal level increases with increasing received signal level up to a certain level, and thereafter is flat as illustrated by curve 6 in FIG. 1. If no AGC is present, the output signal level increases linearly as a function of the input signal level as shown by curve 8 in FIG. 1. With a non-flat AGC circuit, the output signal level increases linearly with increasing input signal level up to a certain level, and thereafter increases at a lesser rate with respect to the input signal level, as shown by curve 7 in FIG. 1. A non-flat AGC characteristic is helpful to the operator of such a receiver in that tuning the receiver is easier, because the frequency at which maximum signal strength is received (i.e., the center of the band) is more easily sensed. Non-flat AGC also minimizes interstation noise and requires a lower value of maximum gain in a receiver's IF circuitry.
If a non-flat AGC characteristic is used in connection with an AM stereo receiver of the type described in the prior Kahn patent, which is illustrated in FIG. 2, the changing level A of the envelope detector output, corresponding to the carrier level at the detector (which is a function of tuning and received signal strength), will result in an improper correction signal being supplied to the inverse modulator 22 and, therefore, improper distortion correction. In FIG. 2, and elsewhere hereinafter, X.sub.+ represents the stereo sum signal (L+R) and X.sub.- represents the stereo difference signal (L-R).
It is, therefore, an object of the present invention to provide a receiver which is generally of the distortion correcting type disclosed in the aforementioned Kahn patent, but which has a non-flat AGC characteristic.